tree-optimization/116460 - ICE with DCE in forwprop

tree-optimization/116460 - ICE with DCE in forwprop

The following avoids removing stmts with defs that might still have
uses in the IL before calling simple_dce_from_worklist which might
remove those as that will wreck debug stmt generation.  Instead first
perform use-based DCE and then remove stmts which may have uses in
code that CFG cleanup will remove.  This requires tracking stmts
in to_remove by their SSA def so we can check whether it was removed
before without running into the issue that PHIs can be ggc_free()d
upon removal.  So this adds to_remove_defs in addition to to_remove
which has to stay to track GIMPLE_NOPs we want to elide.

	PR tree-optimization/116460
	* tree-ssa-forwprop.cc (pass_forwprop::execute): First do
	simple_dce_from_worklist and then remove stmts in to_remove.
	Track defs to be removed in to_remove_defs.

	* g++.dg/torture/pr116460.C: New testcase.